explain and test reverse_view!

Author: Lilith Hafner

base/sort.jl

@@ -593,11 +593,11 @@ function partition!(t::AbstractVector, lo::Integer, hi::Integer, o::Ordering, v:
 end
 
 # Needed for bootstrapping because `view` is not available.
+# `reverse_view!(v, lo, hi)` has the same side effect as `reverse!(view(v, lo:hi))`
 function reverse_view!(v, lo, hi)
     for i in 0:div(hi-lo-1, 2)
         @inbounds v[lo+i], v[hi-i] = v[hi-i], v[lo+i]
     end
-    v
 end
 
 function sort!(v::AbstractVector, lo::Integer, hi::Integer, a::PartialQuickSort, o::Ordering, t::AbstractVector=similar(v), swap=false, rev=false)

test/sorting.jl

@@ -41,6 +41,25 @@ end
     @test Base.Sort.select_pivot([3, 2, 1], 1, 3, Reverse) == 2
 end
 
+@testset "reverse_view!" begin
+    a = [3, 1, 2, 5, 4]
+    b = copy(a)
+    rv!(v, lo, hi) = (reverse!(view(v, lo:hi)); v)
+    @test a == b
+    Base.Sort.reverse_view!(a, 1, 5)
+    rv!(b, 1, 5)
+    @test a == b
+    Base.Sort.reverse_view!(a, 2, 5)
+    rv!(b, 2, 5)
+    @test a == b
+    Base.Sort.reverse_view!(a, 1, 1)
+    rv!(b, 1, 1)
+    @test a == b
+    Base.Sort.reverse_view!(a, 3, 1)
+    rv!(b, 3, 1)
+    @test a == b
+end
+
 @testset "sort" begin
     @test sort([2,3,1]) == [1,2,3] == sort([2,3,1]; order=Forward)
     @test sort([2,3,1], rev=true) == [3,2,1] == sort([2,3,1], order=Reverse)